1. Field of the Invention
The present invention relates to an apparatus for focusing a light beam from a light source such as a semiconductor laser onto a rotating disk-shaped recording medium (hereinafter, referred to as an xe2x80x9coptical diskxe2x80x9d) so as to record/reproduce signals on/from the optical disk. More particularly, the present invention relates to a tracking control for positioning the light beam along a track on the optical disk.
2. Description of the Related Art
With a conventional optical disk apparatus, a signal is reproduced from an optical disk by irradiating the optical disk with a light beam of a relatively small but constant light amount so as to detect the reflected light from the optical disk whose intensity has been modulated by the optical disk. A signal is recorded on the optical disk by writing information on a recording material film of the optical disk with a light beam while modulating the intensity thereof according to the signal to be recorded (e.g., Japanese Laid-Open Publication No. 52-80802).
A read-only optical disk is produced by recording a plurality of information pits on the disk in a spiral pattern. A recordable/reproducible optical disk is produced by providing an optically recordable/reproducible material film through a process such as vapor deposition on a surface of a substrate which includes spiral concave/convex tracks thereon.
To properly record information on an optical disk or reproduce the recorded information from the optical disk requires a focusing control and a tracking control. The focusing control is for controlling the optical disk in a direction normal to the optical disk surface (hereinafter, referred to as the xe2x80x9cfocusing directionxe2x80x9d) so that the light beam is always in a predetermined focused state at the recording material film. The tracking control is for controlling the optical disk in a radial direction of the optical disk (hereinafter, referred to as the xe2x80x9ctracking directionxe2x80x9d) so that the light beam is always positioned along a predetermined track.
A conventional tracking control for an optical disk will be described with reference to FIG. 8. A disk-shaped optical disk 1 is rotated by a disk motor 50. An optical head 10 includes a semiconductor laser 11, a coupling lens 12, a polarization beam splitter 13, a xc2xc wave plate 14, a focusing actuator 16, a tracking actuator 17, a detection lens 18, a cylindrical lens 19 and a 4-divided photodetector 20.
The optical head 10 can be traversed by a traverse motor 43 in the tracking direction. A light beam generated from the semiconductor laser 11 is collimated by the coupling lens 12, passes through the polarization beam splitter 13 and the xc2xc wave plate 14, and is then focused by a focusing lens 15 on the optical disk 1.
The light beam is reflected by the optical disk 1, passes through the focusing lens 15 and the xc2xc wave plate 14, and is then reflected by the polarization beam splitter 13. Thereafter, the reflected light passes through the detection lens 18 and the cylindrical lens 19 so as to be incident upon the 4-divided photodetector 20.
The focusing lens 15 is supported by an elastic body (not shown). The focusing lens 15 is moved in the focusing direction by applying a current to the focusing actuator 16 and in the tracking direction by applying a current to the tracking actuator 17.
The photodetector 20 detects a light amount signal and sends the detected light amount signal to a focusing error detector (hereinafter, referred to as the xe2x80x9cFE generatorxe2x80x9d) 30 and to a tracking error detector (hereinafter, referred to as the xe2x80x9cTE generatorxe2x80x9d) 40.
Using the light amount signal from the photodetector 20, the FE generator 30 calculates an error signal (hereinafter, referred to as an xe2x80x9cFE signalxe2x80x9d) which indicates the focused state of the light beam at the information surface of the optical disk 1, and sends the FE signal to the focusing actuator 16 via a focusing linear filter (hereinafter, referred to as the xe2x80x9cFc linear filterxe2x80x9d) 31. The focusing actuator 16 controls the focusing lens 15 in the focusing direction so that the light beam is focused on the recording surface of the optical disk 1 in a predetermined state. Thus, the focusing control is performed.
Using the light amount signal from the photodetector 20, the TE generator 40 also calculates an error signal (hereinafter, referred to as a xe2x80x9cTE signalxe2x80x9d) which indicates the positional relationship between the light beam and an intended track on the optical disk 1, and sends the TE signal to the tracking actuator 17 via a tracking linear filter (hereinafter, referred to as the xe2x80x9cTk linear filterxe2x80x9d) 41.
The tracking actuator 17 controls the focusing lens 15 in the tracking direction so that the light beam properly follows a track. The tracks of the optical disk 1 exist over a large area of the optical disk 1, extending from the inner periphery to the outer periphery of the optical disk 1. Therefore, the focusing lens 15 needs to be movable over a large extent in order to irradiate the intended track with the light beam.
Since the motion range of the tracking actuator 17 is limited, the optical head 10 needs to be driven in the tracking direction. Therefore, a drive signal output from the Tk linear filter 41 to the tracking actuator 17 is sent to the traverse motor 43 via a traverse linear filter 42, an average calculator 45 and a pulse generator 44 so as to move the optical head 10 in the tracking direction through the rotation of the traverse motor 43.
Thus, the optical head 10 moves in the tracking direction so that the drive signal to the tracking actuator 17 approaches zero or, in other words, so that the focusing lens 15 takes a normal position with respect to the optical head 10. By the two devices, i.e., the tracking actuator 17 and the traverse motor 43, operating as described above, the light beam follows a track on the optical disk 1. Thus, the tracking control is performed.
Typically, as compared with the tracking actuator 17, the traverse motor 43 is only responsive to an input signal having a relatively low frequency. The traverse linear filter 42 extracts a low-band component of the signal from the Tk linear filter 41, for which the traverse motor 43 can sufficiently follow the track, through a low-pass filter having a low-pass characteristic as illustrated in FIG. 9. Thus, the traverse motor 43 is driven by the extracted low-band component.
To move the optical head 10 by the traverse motor 43 requires a driving force which overcomes the frictional force of the traverse motor 43 itself or the frictional force of a mechanism for traversing the optical head 10.
Moreover, when the optical disk 1 has some eccentricity, the drive signal from the Tk linear filter 41 includes eccentricity components so that the light beam can properly follow the track. When the optical disk 1 rotates at a high speed, it is difficult for the traverse motor 43 to follow the eccentricity components of the drive signal. Therefore, it is necessary to drive the traverse motor 43 to eliminate an influence of the eccentricity.
In an optical disk apparatus 800 illustrated in FIG. 8, a signal from the traverse linear filter 42 is sent to the traverse motor 43 via the average calculator 45 and the pulse generator 44.
An operation of the optical disk apparatus 800 will be described with reference to FIGS. 10A to 10D. FIG. 10A illustrates a signal from the disk motor 50, FIG. 10B illustrates a signal from the traverse linear filter 42, FIG. 10C illustrates a signal from the average calculator 45, an FIG. 10D illustrates a signal from the pulse generator 44.
Referring to FIG. 10A, the disk motor 50 outputs one cycle of a square wave signal (hereinafter, referred to as the xe2x80x9cFG signalxe2x80x9d) for each revolution thereof. Referring to FIG. 10C, the average calculator 45 calculates the average value of the signal output from the traverse linear filter 42 during a time period from the rising edge t1 to the rising edge t2 of the FG signal, and outputs the average value for the next time period from the rising edge t2 to the rising edge t3.
Since the average value is calculated for each revolution of the optical disk 1, the signal from the average calculator 45 is not influenced by the eccentricity of the optical disk 1. Referring to FIG. 10D, the pulse generator 44 outputs a pulse signal having a wave height and a pulse width which are respectively predetermined to be sufficient for driving the traverse motor 43, when the signal from the average calculator 45 exceeds a predetermined level SL (see FIG. 10C) (e.g., Japanese Laid-Open Publication No. 7-98877).
In the above-described conventional tracking control, the pulse width and the wave height of the drive signal to the traverse motor 43 are both fixed values. Therefore, as the frictional force of the traverse motor 43 itself or the frictional force of the mechanism for traversing the optical head 10 increase over time, the driving force of the traverse motor 43 may not overcome such frictional forces.
Moreover, since the average value is calculated for each revolution of the disk motor 50, the response speed of the traverse motor 43 is determined by the number of revolutions of the disk motor 50. Thus, the response speed of the traverse motor 43 decreases as the number of revolutions of the disk motor 50 decreases.
According to one aspect of this invention, there is provided an optical disk apparatus for irradiating a track on an information carrier on which information is recorded with a light beam so as to reproduce information from the track. The optical disk apparatus includes: a tracking error detector for detecting a tracking error signal representing a displacement between the light beam and the track; a fine movement section for moving the light beam in a substantially radial direction of the information carrier; a coarse movement section for moving the fine movement section in the substantially radial direction of the information carrier; a tracking controller for controlling the fine movement section and the coarse movement section based on the tracking error signal detected by the tracking error detector so that the light beam is positioned along the track; and an eccentricity detector for detecting an amount of eccentricity of the track on the information carrier. The tracking controller controls the coarse movement section based on the amount of eccentricity detected by the eccentricity detector.
In one embodiment of the invention, the eccentricity detector detects the amount of eccentricity based on an output of the tracking controller.
In one embodiment of the invention, the tracking controller includes: a fine tracking controller for controlling the fine movement section based on the tracking error signal so that the light beam is positioned along the track; and a coarse tracking controller for controlling the coarse movement section so that an amount of movement of the fine movement section by the fine tracking controller is zero on average.
In one embodiment of the invention, the eccentricity detector detects the amount of eccentricity based on an output of the coarse tracking controller.
In one embodiment of the invention, the eccentricity detector detects the amount of eccentricity based on an amplitude of an alternating current component of a control signal which is used by the coarse tracking controller for controlling the coarse movement section.
In one embodiment of the invention, the coarse tracking controller includes: a traverse linear filter for outputting a first drive signal for driving the coarse movement section based on an output of the fine tracking controller; and a traverse drive generator for outputting a second drive signal for driving the coarse movement section based on the first drive signal output from the traverse linear filter and based on the amount of eccentricity detected by the eccentricity detector. The eccentricity detector detects the amount of eccentricity based on the first drive signal output from the traverse linear filter.
In one embodiment of the invention, the tracking controller includes a switch for inactivating a control of the coarse movement section. The eccentricity detector detects the amount of eccentricity while inactivating the control of the coarse movement section by the switch during an operation of the tracking controller.
In one embodiment of the invention, the eccentricity detector detects the amount of eccentricity based on an output from the fine tracking controller.
In one embodiment of the invention, the eccentricity detector detects the amount of eccentricity based on an amplitude of an alternating current component of a control signal which is used by the fine tracking controller for controlling the fine movement section.
In one embodiment of the invention, the fine tracking controller includes a tracking linear filter for outputting a control signal for controlling the fine movement section. The eccentricity detector detects the amount of eccentricity based on an amplitude of an alternating current component of the control signal which is output from the tracking linear filter.
In one embodiment of the invention, the eccentricity detector detects the amount of eccentricity based on an output of the tracking error detector.
In one embodiment of the invention, the eccentricity detector detects the amount of eccentricity based on an amplitude of an alternating current component of the output of the tracking error detector.
In one embodiment of the invention, the coarse tracking controller includes: a traverse linear filter for outputting a first drive signal for driving the coarse movement section based on an output of the fine tracking controller: and a traverse drive generator for outputting a second drive signal for driving the coarse movement section based on the first drive signal output from the traverse linear filter and based on the amount of eccentricity detected by the eccentricity detector.
In one embodiment of the invention, the fine tracking controller includes a tracking linear filter for outputting a control signal for controlling the fine movement section.
In one embodiment of the invention, the eccentricity detector includes a dead zone width calculator for calculating a dead zone width representing a range in which a value of a drive signal for driving the coarse movement section is substantially zero, based on the amount of eccentricity. The tracking controller controls the coarse movement section based on the dead zone width calculated by the dead zone width calculator.
In one embodiment of the invention, the eccentricity detector includes an offset calculator for calculating a drive offset to be added to a drive signal for driving the coarse movement section based on the amount of eccentricity. The tracking controller controls the coarse movement section based on the drive offset calculated by the offset calculator.
Thus, the invention described herein makes possible the advantages of: (1) providing an optical disk apparatus capable of driving a traverse motor even when the frictional force of the traverse motor itself or the frictional force of the mechanism for traversing the optical head increases over time; and (2) providing an optical disk apparatus where the traverse motor has a desirable response speed even when the number of revolutions of the disk motor is small.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.